Fix binary expression instanceof in arrow function expression (#913)

* Add fix for parens of BinaryExpression with instanceof op embedded into ArrowFunctionExpression.

* Add new test 🚀.

src/fast-path.js

@@ -305,6 +305,13 @@ FPp.needsParens = function(assumeExpressionContext) {
         return true;
       }
 
+      if (
+        node.operator === "instanceof" &&
+        parent.type === "ArrowFunctionExpression"
+      ) {
+        return true;
+      }
+
     case "LogicalExpression":
       switch (parent.type) {
         case "CallExpression":

tests/arrows/__snapshots__/jsfmt.spec.js.snap

@@ -9,6 +9,7 @@ export default (() => {})();
 new (() => {});
 if ((() => {}) ? 1 : 0) {}
 let f = () => ({}())
+let a = () => ({} instanceof a);
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 (a => {}).length;
 typeof (() => {});
@@ -19,6 +20,7 @@ new (() => {})();
 if ((() => {}) ? 1 : 0) {
 }
 let f = () => ({}());
+let a = () => ({} instanceof a);
 "
 `;
 

tests/arrows/arrow_function_expression.js

@@ -6,3 +6,4 @@ export default (() => {})();
 new (() => {});
 if ((() => {}) ? 1 : 0) {}
 let f = () => ({}())
+let a = () => ({} instanceof a);